Fixed off-by-one error in checking length of abstract namespace Unix sockets

[derickr]

I was trying to use PHP to talk to Xdebug's new out-of-band control sockets, which use the abstract Unix domain socket namespace (socket names starting with a \0 character).

Xdebug uses names that are the maximum allowed length for these socket names (108 bytes).

However, when I tried to run:

<?php
$name = "\0xdebug-ctrl.2078073yxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
echo strlen($name), "\n";
$f = stream_socket_client("unix://$name");

I got the following output and error:

108

Notice: stream_socket_client(): socket path exceeded the maximum allowed length of 108 bytes and was truncated in Standard input code on line 2

But the path is exactly 108 characters, not more. It turned out that the check for this maximum length is wrong.

No new test can easily be created, as the 108 is implementation specific, and ext/standard/tests/streams/bug60106.phpt already covers it.

With this change, I can now correctly connection to these sockets.

[ndossche]

The way I'm reading https://man7.org/linux/man-pages/man7/unix.7.html it seems that the >= check should be used for non-abstract namespace sockets while > should be used for abstract namespace sockets.

I'm specifically referring to the section "Three types of address are distinguished in the sockaddr_un structure:"
which says:

a UNIX domain socket can be bound to a null-terminated
filesystem pathname using bind(2).

This means that we must take into account the space for the NUL terminator in the case of a non-abstract namespace socket.

EDIT: I think you need something like this instead https://gist.github.com/nielsdos/ee33321f88632a8c228e6d3c0efcdf5b

[iluuu1994]

I read it the same way as @nielsdos, his Gist looks correct to me.

[iluuu1994]

It doesn't seem like either macOS nor FreeBSD support abstract socket names. So you should likely skip this test for everything but Linux.

[derickr]

Yes, I forgot about that. I'm sad that OSX/FBSD don't support it though.